Firefox randomly switches windows

For the past several versions, when I have multiple open Firefox windows, focus will randomly switch to one of the non-active windows.

I frequently have multiple Firefox windows open, some with more than one tab. All the windows are maximized. In the visible (top-most) window, I may be scrolling the screen, or moving the mouse around, hovering over various objects. Suddenly, one of the other, inactive windows appears over the top of the window I was currently working in. This happens intermittently, and I have seen no pattern to it.

I'm not sure if I ever saw this on Windows 7 (or XP), but it happens several times every day on Windows 8.0/8.1. I've seen it on FF 28.0 to current, and maybe a couple of versions earlier than this. It doesn't seem to be affected by whatever else is running at the time.

Just to add my 2p worth, I have W8.1 and FF30 and have never had the problem (even with older versions). I suppose it's the price we pay for having massively modifiable hardware and software - my set-up is different to your set-up which is different to his set-up and her set-up, so trying to pin down these problems can be... well problematical. At least if you know some others share your issue and some don't, it's probably in your set-up rather than a bug.